A predominant number of color picture tubes in use today have line screens and shadow masks that include slit-shaped apertures. The apertures are aligned in columns, and the adjacent apertures in each column are separated from each other by webs or tie bars in the mask. Such tie bars are essential in a the mask, to maintain its integrity when it is formed into a dome-shaped contour which somewhat parallels the contour of the interior of a viewing faceplate of a tube. Tie bars in one column are offset in the longitudinal direction of the column (vertical direction) from the tie bars in the immediately adjacent columns. Because of the pattern of apertures and tie bars, the upper and lower borders of the aperture array are somewhat jagged. Some aperture columns end with tie bars near the border, while other columns end with apertures at the border. When a viewing screen is formed using the mask as a photomaster, the resultant screen also has jagged upper and lower borders. Such jagged borders are esthetically undesirable.
One solution that produces screens with smooth borders is disclosed in U.S. Patent No. 4,300,070, issued to R. H. Godfrey et al. on Nov. 10, 1981. In that patent, the aperture array border is smoothed with a complicated method of lengthening and shortening the pitches of the last two apertures in each column. The method not only made the border geometry smooth, but also equalized the light output at the edges of the screen. Such method was developed for use with masks having a constant tie bar pitch and a projected straight line border on the screen.
Recently, attempts have been made to produce shadow masks wherein the tie bar vertical repeat, or tie bar pitch, is varied. The object of the variation is to produce a constant pitch shadow on the screen to reduce the problem of moire. If the top and bottom borders of the screens of tubes using these masks with varied tie bar pitch are to remain a straight line projections, there is a need for a new border smoothing technique which can be done simply for these masks.